1. Field of the Invention
The present invention relates to a document feeding device of a facsimile system, and more particularly, relates to an automatic document feeding device of a facsimile system which prevents noise and resonance frequency due to mechanical abrasion and vibration when document is conveyed in such a facsimile system in order to optimize its mechanical condition and minimize image resolution deterioration and jitter.
2. Related Art
Conventional facsimile system operates in three modes: transmission, reception, and copying modes. In the transmission mode, an original document is read and the read image data is transmitted to a destination. In the reception mode, the image data sent from another communication system is recorded on a recording medium. In the copying mode, an original document is read and is recorded directly. Each facsimile system typically contains a signal transmission section and a signal reception section. The signal transmission section includes feed rollers for feeding a number of documents one by one to a predetermined station inside the system, transfer rollers for transferring the documents fed by the feed rollers, pressure rollers for closely contacting the surfaces of the documents with an image sensor for scanning the information recorded on the documents and a transmission motor for driving all rollers to permit reading and transmission of the original document. Contemporary document feeders for facsimile systems are disclosed, for example, in U.S. Pat. No. 4,866,531 for Recording Medium Feeding Apparatus issued to Kobori et al., U.S. Pat. No. 5,206,737 for Facsimile Apparatus Using A Single Bi-Directional Motor To Control Feeding Of an Original Document And A Recording Paper With Selection Between Plural Modes Of Operation issued to Sugiyama, U.S. Pat. No. 5,226,639 for Paper Feeding Device For Facsimile Apparatus issued to Kida et al., U.S. Pat. No. 5,369,509 for Document Transferring System For A Facsimile Apparatus issued to Ko, U.S. Pat. No. 5,391,009 for Single Motor Actuation For Automatic Stack Feeder System In A Hardcopy Device issued to Stodder, U.S. Pat. No. 5,523,858 for Facsimile Apparatus issued to Yamada et al., U.S. Pat. No. 5,550,652 for Automatic Document Feeder For A Facsimile Machine issued to Park, and U.S. Pat. No. 5,552,902 for Facsimile Apparatus With Internal Mechanism For Conveying Originals And Recording Paper issued to Kohno.
Generally, when each sheet of document is loaded to the automatic feeding device for data transmission, an image sensor which senses if document is loaded is operated, and feed rollers rotate, so that a friction pad which is located over the document, picks up the sheet of document for a predetermined distance, and then stops. In this point, the user dials a telephone number of a destination communication system, and pushes a start button to transmit the document according to the destination's reception condition. Otherwise, image data of the document is stored in a memory for an automatic transmission. Once the start button is pushed, the transmission motor is driven under control of a central processing unit (CPU) to drive automatic feed rollers of the facsimile system to convey the document picked up for document transmission to a destination communication system. Thereafter, each sheet of document passes through a discharge roller where the trailing edge of the document sheet is sensed by a sensor in order to temporarily disable operation of the step motor. If the document contains several pages, then each time the trailing edge of the document sheet is sensed by the sensor, a clutch of the automatic feeding roller is used to drive the automatic feeding roller to convey the next document sheet.
In advanced facsimile system including automatic document feeding device, the user can transmit a document through any one of three transmission modes, such as a normal transmission mode, a fine transmission mode and a super fine transmission mode. If highest image resolution is required, the super fine transmission mode must be selected. Fine transmission mode is selected for higher image resolution than normal transmission mode for image normal resolution. Generally, if higher image resolution is required, the reading device takes more time to accurately read image data of the document. Thus, when the document is conveyed on the reading device, the feeding speed of the document, via a step motor, must be controlled for the image data of the document to be read accurately. Thus, if the super fine transmission mode or fine transmission mode is set, the motor speed must be considerably lowered. In most facsimile systems available in the market, however, the optimal motor speed of a step motor is generally set at a revolution suitable only for the normal transmission condition. If the user desires to set the facsimile system in either a super fine transmission mode or a fine transmission mode for higher image resolution transmission in which the step motor must be driven at a revolution lower than that required for the normal transmission mode, the step motor fails to operate in its optimal condition. Consequently, mechanical abrasion between gears due to vibration or noise is inevitable. Moreover, gap between the gears is increased and the document which passes through the reading device is conveyed at an unsteady speed. These noted problems attributes to a system deterioration in image resolution of a received document and generation of jitters.